Sky brightness changes during total solar eclipse

Recently I have shown the light level changes during the solar eclipse. The illumination level is strongly related to the sky brightness. Both of them change at the same time throughout the eclipse.
In this article I would like to focus on these changes, having external data from Wolfgang Strickling. Because his observation venue was only about 200 km ahead I decided to use his measurements in terms of the Polish Society of Amateur Astronomers (PTMA) observation point.

1. INTRODUCTION

The sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The sky brightness in general, we can measure with the magnitude per square second of arc units. These units are used to measure surface brightness. The surface brightness measures the brightness per area in the sky. An area in the sky is measured as the angular distance in square degrees or square arcseconds. It refers to the extended objects (like stars, nebulae, etc.), from which the light comes from an extended surface rather than just from a point. Since the light from these objects is spread out over a small area of the sky, astronomers measure their surface brightness. The extended object appearing in the sky at a certain moment must have a bigger surface brightness than the sky for it to be visible to the naked eye. Otherwise, the brighter sky will completely wash out an object with a smaller surface brightness. Before I start to describe the sky brightness changes during the solar eclipse I would like to bring some information about the daily sky brightness in order to the clarification of this issue and easier comparison with eclipse conditions. The sky brightness varies greatly over the day and the primary cause differs as well. Typical sky brightness during the day varies from 1.6-3.8 mag during the midday time hours to 7 Mag around sunrise/sunset. In general, the sky brightness issue is much wider than in solar eclipses only and is to be developed for me in further readings

     2. MEASUREMENT TOOLS

The sky brightness during totality can be measured by a sky quality meter or sky brightness meter. A Sky Quality Meter is used typically by amateur astronomers to quantify the sky glow aspect of light pollution. There are several models of Sky Quality Meter offering different fields of view and various automatic measurement and data logging capabilities. The Sky Quality Meter has become the most common device used to track the evolution of the brightness of the sky from polluted regions to first-class astronomical observatories (Sanchez et al., 2017). The sky brightness meter is a small device, that can be integrated with a weather station as follows Wolfgang’s Strickling solution. This small and handy design autonomous by means of the rechargeable battery is equipped with temperature, brightness, and wind sensor. The newest and the most professional instruments were presented during the Great American Eclipse, where the measurements were done by tracking CCD cameras with color filters, and the wide-angle lens allowed measurements across a wide field of view, recording images every 10s (Bruns, 2018). In the result also the sky color changes have been recorded. Other authors used a tripod-mounted Fujifilm X-E2 camera with a built intervalometer, which proceeded at one frame per minute (Dickson, 2017). In my work, I will focus on Wolfgang’s Strickling instrument, because this is where my data comes from.

     3. MEASUREMENT METHODS

The sky brightness measurements done by Wolfgang Strickling and opened up for me by his courtesy led me to take a detailed insight into this phenomenon. Some of the general information is available on his website. Thankfully I got much detailed .csv data to further proceed.
Because Wolfgang Strickling carried out the observation in Jackson Hole, WY is located only 230 km from our eclipse observation place I literally moved his data into our case. Due to this, I had to take into account the following pivot assumptions:
– The totality period difference on this distance (around 5 sec) has been omitted.
– Weather conditions (mainly aerosol concentration) changes between these 2 places have been also omitted.
– The solar position in the sky (53 degrees for us and around 51,5 degrees altitude with azimuth accordingly 141 and 139) has been also neglected.
Despite the important omissions, that had to come through these results I came to very detailed Conclusions.

According to Wolfgang’s Strickling instrument, the sky brightness measurements don’t cover a whole solar eclipse phenomenon. The data range includes the period from around 15 min before totality to 10 min after, giving 27 min of full measurement. Because the data from extreme moments of the measurement were sampled on an intermittent basis I decided to narrow these analyses down to 6 min before and after totality except for the overall charts giving values up to 10 min after 3^rd contact.
Wolfgang Strickling has measured the sky brightness around the totality in 5 directions: 4 cardinal points and the zenith. These measurements (except the zenith one) were aimed at 15 and 45 degrees above the horizon (Pic. 1). Basically, these angular distances from the horizon are the more haze-dependant, thus their brightness changes more effectively throughout the totality.

Pic. 1 A sky surface brightness measurement directions placed on the fisheye picture taken during the 2017 Greatest American Eclipse. The image was taken before the mid-eclipse, therefore eastern (shadow-out) horizon remains bright. Anyhow at the mid-eclipse, the south-eastern part of the sky appears to be the brightest section. Always a fisheye image allows us to see it when a whole sky sphere is covered (based on Alex Conu, Astrographist.com)

4. SKY BRIGHTNESS CHANGES THROUGHOUT A SOLAR ECLIPSE

Analyzing main plots we can infer the umbral impact on overall sky brightness in more or less all directions. Anyhow, I am going to describe all these changes separately based on my visual observations combined with my colleague’s records and Wolfgang’s Strickling measurements from Jackson Hole.

North-South

The situation in the northern and southern sky was quite straightforward. Because basically, the umbra was moving from the west towards the east the sky brightness changes towards these directions were the most symmetrical against mid-eclipse. However, even here some differences have been recorded because the lunar shadow was moving roughly from WNW to the ESE direction.
The northern part of the sky remained the darkest throughout the whole eclipse phenomena due to the solar position (Pic. 2). I have explained this issue before in this article and also in the light level changes description. Because the lunar shadow approached from the northwest the northern sky brightness drop was more noticeable before 2^nd contact. The results clearly show a quicker Mag/arcsec² increment in 1^st partial phase for 45 deg altitude. The difference before-after mid-eclipse rises up to 0.02 mag for 93,5% obscuration and 0.03 mag for 94% obscuration. For this obscuration level, there is still no difference for the sky at 15 deg altitude, where the difference was recorded for 94,5% obscuration. The biggest northern sky brightness difference has been noticed around 30 sec outside the totality for 99,6% obscuration (Pic. 3). According to the instrument the darkest northern sky was recorded 30 sec before 2^nd contact and continued up to 30 sec after 2^nd contact (Pic. 6). Before mid-eclipse, the northern sky at 15 deg altitude started getting brighter reaching -0.04 mag/arcsec drop. General for altitude 15 deg above the horizon the sky brightness changes remained almost symmetrical with a minimum value of around 30-40 sec before mid-eclipse unlike to higher part of the sky (45 deg) with a mid-eclipse minimum. At the mid-eclipse, the northern sky brightness was 11.32 mag/arcsec for 15 deg and 12.44 mag/arcsec² for 45 deg above the horizon (Pic. 3,5,6).

Pic. 2 North-South sky brightness comparison at the 3 major moments of totality. The southern sky appears to be slightly darker misleadingly, due to the camera response. However, this section contains a much more yellowish tint present all way up from the horizon (footage: Tomasz Rać).

Pic. 3 Sky brightness measurement at the northern section of the sky within 92% obscuration, before-after comparison.

The southern part of the sky’s behavior differed much due to high haze presence and forward scattering occurrence (Pic. 4). A considerable difference is discernable between 15 deg and 45 deg altitude. The southern sky brightness difference is to be seen over the whole measurement period, although a noticeable difference is to be seen for 97% obscuration at 15 deg above the horizon (0.16 mag) and 97,4% obscuration at 45 deg above the horizon. We must know, that southern sky brightness variations were much bigger due to the solar position around 53 deg above the horizon. In terms of this, the changes occurred proportionally to a whole result. At the beginning of the measurement, the 45 deg sky brightness was 3.51 Mag/arcsec² (for 92,5% obscuration). At the mid-eclipse, this value changed to 12.2 Mag. The southern sky at 15 deg above the horizon was a bit darker, due to the bigger angular distance from the Sun. The initial sky brightness value was 6.02 mag and the lowest was 12.37 mag (Pic. 5, 6). The biggest sky brightness difference has been recorded for 99,6% at 15 deg altitude and between 2^nd and 3^rd contact accordingly 1.49 mag for 45 degrees and 1.02 mag for 15 deg above the horizon (Pic. 4). Another curious thing is the inverted brightness change in before-after relation up to 97% (Pic. 4, 5).

Pic. 4 Sky brightness measurement at the southern section of the sky within 92% obscuration, before-after comparison.

I am not sure about the reason here, because the measurements come from a different place. I can guess, that this is because of the solar position movement across the sky. The instrument was heading exactly southward whereas the angular distance to the Sun was decreasing. The totality in Jackson Hole lasted earlier than in Riverton against the astronomical noon. As the angular distance was reduced the forward scattering became more significant, thereby sky brightness increased in the span of the time of measurement.

Pic. 5 Sky surface brightness during the 2017 Greatest American Eclipse at the north and south sections of the sky.

Pic. 6 Sky surface brightness measured during 2017 totality in the south and north directions.

East-West

These sections of the sky were the most interesting because they indicated almost correctly the shadow-in and shadow-out directions (Pic. 7). The sky brightness difference is the most asymmetrical both on the western and eastern sections of the sky. A bigger difference is to be seen for 15 deg altitude both on the western and eastern part of the sky. Because the measurement covers the interval between 92% obscuration only there is no information about the moment when this difference started to occur.
For the western part of the sky, the umbra approaching has been recorded at 94.5% obscuration. The before-after difference value increased rapidly from 0.16 to 0.28 Mag/arcsec² (Pic. 8). Next significant changes occur for 95,5% obscuration with similar values. The sky brightness before-after difference for 45 deg altitude is not as big as for 15 deg probably due to the bigger thickness of the atmosphere, which was affected by the lunar shadow on a long way. At the upper sky, the thinner atmosphere doesn’t incur big brightness changes, hence the before-after difference is less noticeable even in hazy conditions. Anyway here also 94% is a significant moment, where the before-after difference becomes bigger. The biggest values of this difference are to be seen accordingly at 2^nd contact for 15 degrees (0.9 Mag) and at 99.6% obscuration for 45° altitude (0.94 Mag). The reason is that the lower part of the western sky starts to brighten much faster than the upper one after mid-eclipse (Pic. 8, 11). This significant difference persists until mid-eclipse, however, decreases as a normal pattern. At 30-sec before-after mid-eclipse, the values are 0.51 mag for 15 degrees and 0.28 mag for 45 degrees. From the 4^th second of totality to mid-eclipse the lower part of the western sky is brightening twice quicker than the upper part at 45 deg altitude (Pic. 8, 10).

Pic. 7 East-West sky brightness comparison at the 3 major moments of totality. The southern sky appears to be slightly darker misleadingly, due to the camera response. However, this section contains a much more yellowish tint present all way up from the horizon (footage: Tomasz Rać)

Pic. 8 Sky brightness measurement at the western section of sky within 92% obscuration, before-after comparison.

The eastern part of the sky represents the opposite behavior, but due to forward scattering, the overall result is slightly different. First of all the sky brightness changes, and before-after differences occurred likewise on the western part. We can clearly see, that as the mid-eclipse approaches the lower part of the sky is darkening around twice as fast as the upper part. This situation happened from the 2^nd contact giving values accordingly of 0.94 Mag for 15 deg and 0.57 mag for 45 deg whilst 40 seconds later these values state 0.43 mag for 15 deg and 0.19 mag for 45 degrees altitude (Pic. 9, 11). Secondly, the perfect view of the dark shadow cone has been reported through these measurements. The biggest before-after difference has been recorded for 99,6% obscuration, which clearly reflects the dark shadow cone position. The difference value is 1.43mag/arcsec² for 15 deg and 0.02mag/arcsec² for 45 deg altitude giving accordingly 10.83 and 10.62 mag/arcsec² (Pic. 9, 10). Because of the not-big brightness difference, the shadow-out sky featured quite a uniform dark hue up to 40-50 sec after 3^rd contact. Next, this hue was brighter and brighter. The before-after difference is getting lower as proceed further from the totality and shows, that the lower part of the sky is brightening quicker up to 97.4% obscuration. This is the moment when the grey shadow cone is definitely gone, leaving a bright sky with blueness washed out due to forward scattering. The last umbral presence can be spotted at 95% obscuration at both 15° and 45° altitudes. By the end of the measurement, the before-after brightness difference for the upper part of the sky is vanishingly small with 0.06 Mag/arcsec² magnitude only giving the value 6.53 Mag/arcsec² (Pic. 9, 10).

Pic. 9 Sky brightness measurement at the eastern section of sky within 92% obscuration, before-after comparison.

Pic. 10 Sky surface brightness during the 2017 Greatest American Eclipse at the eastern and western sections of the sky.

Pic. 11 Sky surface brightness measured during the 2017 totality at eastern and western directions.

Zenith sky

Zenith sky reflected similar brightness changes to the upper part of the western sky. Basically, there is no difference for obscuration of less than 98,7% (Pic. 12). A huge before-after brightness difference is to be noticed for 99,6% obscuration with 1.03 Mag/arcsec² value (Pic. 13). It was the moment when the 30s before 2^nd contact lunar shadow started to cover the zenith sky. Unlike 99,6% obscuration, there is almost no difference for 30 seconds against mid-eclipse. The zenith was one of the darkest parts of the sky during totality and for sure the darkest measured the direction with a value of 12.85 Mag/arcsec². In the plot, we can clearly see how the zenith sky starts to brighten immediately after the mid-eclipse (Pic. 14).

Pic. 12 Sky brightness measurement at zenith sky within 92% obscuration, before-after comparison.

Pic. 13 Sky surface brightness during the 2017 Greatest American Eclipse at zenith sky.

Pic. 14 Sky surface brightness measured during 2017 totality at zenith.

5. OVERALL SKY BRIGHTNESS COMPARISON – TOTALITY

As a result of all measurements, I have prepared an outlook on all directions at once. My graphs contain a totality period only, because this period is the most important, showing the most significant difference in sky brightness between its particular regions.

Pic. 15 The sky during the total phase of the eclipse in all cardinal directions, captured just before the mid-eclipse.  Olympus A510, 14mm, f/2.2m 1/25 with EV +1,5.

Watching the picture above we can see a different sky brightness in all cardinal directions. The southern and eastern sky was veiled by clouds, however, we can still spot a slightly bigger brightness, especially near the horizon. Moreover eastern sky color contains a yellowish tint, which is related to secondary scattered light, coming from the illuminated area. The northern and western part of the sky looks darker.
Is also worth seeing the charts below, which show a measured sky brightness at 3 key moments of the totality (Pic. 16 – 18), and see how the Mag/arcsec² values changed within this time.

Pic. 16 – 18 Sky surface brightness values (Mag/arcsec²) for major moments of totality, where: 16 – – 2^nd contact; 17 – mid-eclipse; 18 – 3^rd contact.

Comparing these 3 charts above we can see, that definitely the darkest part of the sky throughout the eclipse is the zenith. This is understandable because looking towards the zenith we are looking through the thinnest part of the Earth’s atmosphere.
The brightest section of the sky appears to be located near the Sun, however, an exception from this rule occurs at mid-eclipse when a southern sky is one of the darkest. One thing, that can explain this issue is the presence of a high-level cloud, which dropped the values. I have no information about cloud coverage from Wolfgang’s Strickling measurement venue, although these results clearly show, that some cloudiness at the southern part of the sky could take place, especially on lower altitudes above the horizon (which was darker, than the upper one). Another reason is the Sun brightness itself, which during the totality drops down to -11,5 Mag, lifting up the local sky brightness slightly, which can be visible on the graph (Pic. 17). A sky brightness on the northern part of the sky changed seamlessly and quite symmetrically against umbral contacts. The lower part of the sky remained much brighter than the upper one. Both western (shadow-in) and eastern (shadow-out) sections of the sky changed their brightness significantly and rapidly due to shadow movements.

6. CONCLUSIONS

The sky brightness during a total solar eclipse has been measured by Wolfgang Strickling and next moved on my observation position located 230km eastwards from an initial place. Despite the omission of some factors, the results were favorable. It found, that umbral movement across the sky is clearly visible and impacts significantly local sky brightness.
At the very beginning of the measurement at 77,2% obscuration, the sky surface brightness was from 1.86 Mag at south 45 deg altitude (close to the Sun) to 6.16 Mag/arcsec² at west 15 deg altitude. The most detailed measurement includes the period before-after mid-eclipse down to 92% obscuration. For 92% obscuration, the sky surface brightness varies between 3.42 mag/arcsec2 to 7.1 mag/arcsec² at a west 45 deg altitude. Charts shown in this article indicate important moments of sky surface brightness changes during the near-totality period. Analyzing every section of the sky separately. For the shadow-in sky, the most important moment occurred at 94% obscuration, when the sky’s surface brightness started to fall down faster. Next, the moment 30 sec before mid-eclipse was the darkest for the western part of the sky with values 11.43 Mag/arcsec² for 15 deg and 12.55 Mag/arcsec² for 45 deg altitude (2^nd the darkest section of the sky during totality). The opposite situation could be observed in the shadow-out sky with the darkest moment 30 sec before 3^rd contact, where the sky surface brightness reached 11.33 Mag/arcsec² for 15 deg and 12.18 Mag/arcsec² for 45 deg. The most remarkable moment for the shadow-out sky was a grey lunar shadow cone present just after 3^rd contact up to 40-50 seconds ahead. At obscuration, 97,4% of this shadow cone was gone. It can be seen decreasing before-after sky brightness difference down to this moment. At 95% obscuration shadow presence in the sky disappears. Sky surface brightness remains similar at both measured altitudes due to backward light scattering on haze. The northern and southern part of the sky was more symmetrical, however, even here we could spot the difference. The northern sky was a bit darker before 2^nd contact. Southern sky was more affected by the close solar position and possibly its movement across the sky throughout the eclipse. Important sky surface changes on the northern side are to be seen at 98% obscuration. The southern part of the sky changes surface brightness rapidly from 97% obscuration at 15 degrees and 97.4% obscuration at 45 degrees altitude. The moment of the umbra approaching the southern sky was clearly reflected in the measurements as well as shadow cone presence down to 99.6% obscuration after mid-eclipse. Also for the zenith sky, the most important moment was 30 seconds before totality when the umbra covered it. At the mid-eclipse, the zenith sky was the darkest part of the sky with a 12.85 Mag/arcsec² value. The brightest section of the sky at the mid-eclipse was north at 15 deg altitude with 11.32 mag/arcsec².
The result applies to typical total solar eclipse observation during mid-day time hours when high aerosol concentration takes place. There are many other situations, where this kind of measurement could be carried out. I mean especially the horizontal kind of the totality when umbra spreads throughout the whole sky dividing the atmosphere into 2 separate bright sections likewise when a solar eclipse occurs below the horizon. The sky surface brightness changes can be also significant during deep annular eclipses and deep partial eclipses, where Solar disk obscuration reaches the values given. I hope, that my aim of observation will be an inspiration for other eclipse chasers traveling around the world and that someone will undertake similar measurements in future eclipses.

References:

1. Bruns DG, Bruns RD, 2018, Sky brightness and colour measurement during the 21 August 2017 total solar eclipse (in:) Applied Optics, vol. 57 (16) p. 4590-4594
2. Dickson B., 2017, Observation of the sky brightness and colour changes during total solar eclipse of 21 August 2017, (in:) MNASSA, vol. 76, no 9-10, p.256 – 262
3. Sanchez de Miguel A., et al., 2017, Sky Quality Meter measurements in a colour changing world, (in:) Monthly Notices of the Royal Astronomical Society, vol. 2014, p.1-15

Read also:

1. Light level changes during the total solar eclipse
2. Light scattering in the atmosphere during the total solar eclipse
3. Visual range changes during a solar eclipses
4. First time in the shadow